PopupMenu horizontal margins in the editor theme became too low since 4.3.dev3

[Calinou]

Tested versions

• Reproducible in: 4.3.dev3, 4.3.dev4, 4.3.dev fe01776
• Not reproducible in: 4.2.1.stable, 4.3.dev2

System information

Godot v4.3.dev (fe01776) - Fedora Linux 39 (KDE Plasma) - Wayland - GLES3 (Compatibility) - NVIDIA GeForce RTX 4090 (nvidia; 545.29.06) - 13th Gen Intel(R) Core(TM) i9-13900K (32 Threads)

Issue description

PopupMenu horizontal margins in the editor theme became too low since 4.3.dev3. This makes popups feel less consistent with the rest of the editor theme (and most other applications).

Look at the space between the edge of the popup and the New Scene text:

4.3.dev1	4.3.dev4

Back-and-forth animation to compare:

I bisected the regression to dc3b07e (#87085).

While we're at it, we may want to further increase this horizontal margin to make it closer to most other applications (example with a Qt dropdown on KDE Breeze theme):

Steps to reproduce

• Open any popup or dropdown in the editor.

Minimal reproduction project (MRP)

N/A